no sound playing through the Aux port

I installed a Pioneer DEH-P3700mp HU into my car, and got an ip-bus to aux connector installed. The HU runs well. But there was no sound when I connected my mp3 player to the AUX port.  I connected the Mp3 player's earphone output directly to the rca cable.  The aux input resistance is much higher than the resistance of earphone output. is this the reason of no sound? Does any one have such experience? Thanks a lot.

Have you turned on the Aux input in the setup menu? You won't get any audio unless you turn it on there.